As will be readily appreciated, crystal oscillators or crystals, which are found in many electrical circuits, are devices that are fabricated to resonate at predefined frequencies in response to applied voltages. For example, the ubiquitous color burst crystal resonates at a frequency of 3.57954 megahertz (MHz) and may be found in most televisions and radios.
Many systems and circuits utilize crystal oscillators to provide a clock reference representative of relative time. For example, microprocessors and microcontrollers typically utilize crystal oscillators to derive system clocks that control the rate at which data is read by input/output ports and/or the rate at which programming instructions are executed.
Communication systems and components such as telecommunications infrastructure and mobile units use crystal oscillators to generate one or more frequencies that are useful in producing radio frequency (RF) signals onto which information to be broadcast and received is imparted. For example, a crystal oscillator is usually used to generate a master reference clock that is used to synchronize information exchange between telecommunications infrastructure and mobile units.
Crystal oscillators all have tolerance ranges associated with their resonant frequency. The difference between the ideal resonant frequency of a crystal oscillator and the actual operating frequency of the crystal oscillator is referred to as the frequency offset of the crystal oscillator. For example, a crystal oscillator may be specified to have a tolerance between −10 parts-per-million (PPM) and +10 PPM at an operating temperature of 25° C. However, it is a rare situation in which a crystal oscillator is operated at the specified 25° C. temperature. Accordingly, in practice, the actual operational frequency of a crystal oscillator may vary outside the specified 25° C. tolerance. Therefore, there is almost always a frequency offset in a crystal oscillator.
Some applications, such as communications systems, require a high degree of crystal oscillator precision (i.e., very low frequency offset) to prevent interference with neighboring communication channels. To control more precisely the resonant frequency of a crystal oscillator, some communication systems utilize a digitally controlled crystal oscillator (DCXO) system in conjunction with a crystal. A DCXO system typically includes a processing portion that monitors the resonant frequency produced by a crystal oscillator and adjusts the resonant frequency of the crystal oscillator by changing the control code applied to the DCXO circuit. The change in applied code forces a change in the capacitive loading on the crystal oscillator thereby modifying the operating frequency of the crystal oscillator. The capacitive loading is typically achieved via one or more capacitors with switches. The Switches are controlled by the digital interfaces that accept the DCXO codes and connect or disconnect capacitors in response to the DCXO codes, as required.
DCXO systems can be easily tuned to a desired frequency and experience low levels of phase noise interference. However, DCXO systems require significant time to reach a desired frequency at startup and during frequency changes. In other words, immediately after startup, and/or during a frequency change, the signal output by the DCXO will not match the desired signal.